In internal combustion engines, an engine crankshaft is the primary source of torsional load. A crank gear is typically mounted on one end of the crankshaft for driving the engine gear train which includes the cam gear and camshaft among other gear driven elements. Throughout operation of the engine, the torque applied to the crankshaft varies due to, for example, the periodic firing of the engine's cylinders, the engagement and disengagement of driven elements such as a transmission, and the starting, stopping and variations in the speed of rotation of the crankshaft. These torque variations create torsional vibrations which may be transmitted to the gear train of the engine often undesirably causing increased noise, premature engine wear and thus reduced gear life and possibly engine, failure. In addition, since the engine drive gear train is used to drive engine components which are critical to proper engine operation, torsional vibrations transmitted through the gear train may adversely affect engine operation, such as the accuracy of fuel injection timing.
In a conventional engine, a flywheel typically is attached to the rear end of the crankshaft adjacent a coupling/clutch assembly for connection to a driven unit while a damper is attached to the front end of the crankshaft. The flywheel functions to smooth out the power surges generated by the periodic firings in the cylinders thus assisting in minimizing torsional vibrations. Likewise, the damper functions to dampen out or reduce vibrations by dissipating energy. However, while flywheels and dampers achieve some success, torsional vibrations continue to be a significant problem. These torsional vibrations are especially evident in front gear train engines wherein the engine timing gear train is connected to a crank gear mounted on the free end, or front, portion of the crankshaft. For example, the applicant has noted that the inertia associated with the flywheels of prior art couplings tend to cause the node of the first mode of crankshaft torsional vibration to be located in the vicinity of the flywheel, near the rear portion of the crankshaft. The node point corresponds to zero torsional vibration induced motion of the crankshaft of a particular mode. Locating the node near the flywheel has the effect of maximizing the amplitude of the torsional vibration experienced by the front end portion of the crankshaft. Since the timing gear train, and perhaps accessory drives, are typically connected to the front end portion of the crankshaft, the relatively large amplitude of torsional movement of the front end portion of the crankshaft creates undesirable torsional vibrations in the timing gear train. The deleterious effects of torsional vibrations on the gear train are becoming an even greater concern as the demands on the timing gear train, such as higher injection pressure, are increased.
One possible solution to the node positioning problem is to attach the crank gear/timing gear arrangement to the rear end of the crankshaft instead of the front end, i.e. a rear gear train engine. However, a rear gear train mounting arrangement results in various significant drawbacks compared to the front gear train engine. For example, a rear gear train arrangement results in a larger engine incapable of installation in many applications thereby creating complex packaging issues. In addition, accessibility to the engine gear train connection and other engine components is substantially reduced thus increasing the costs and down time associated with engine service, including repair and maintenance.
U.S. Pat. No. 4,617,884 to Allen discloses a torsional vibration isolator device including a flywheel resiliently mounted on the rear end of a crankshaft. The flywheel is thus effectively isolated from the crankshaft to create a natural frequency of torsional vibration of the system to a frequency well below a normal operating range of the engine. However, the assembly results in the node point of zero motion of the crankshaft being located on the rear portion of the crankshaft, an undesirable distance from a front-mounted timing gear train drive.
U.S. Pat. No. 5,303,681 to Crofts discloses a torsional tunable coupling for a diesel engine, including a flywheel connected to a crankshaft which effectively moves the node to the center of the crankshaft. However, the flywheel is rigidly connected to the crankshaft. Also, by using a flywheel having a very low inertia, this arrangement is unlikely to effectively control the rigid body motion of the crankshaft, thus failing to sufficiently prevent undesirable torsional vibration from being transmitted to the driven unit, i.e. transmission.
U.S. Pat. No. 2,880,626 to Nallinger discloses a crankshaft assembly including flywheel inertia mounted on the rear portion of the crankshaft, auxiliary flywheel inertia mounted on the front portion of the crankshaft and an oscillation absorber mounted on the front portion of the crankshaft. The auxiliary flywheel inertia and the oscillation absorber are designed to operate together to reduce the amplitude of torsional vibrations. However, this patent no where suggests shifting the nodal point of first torsional vibration toward a crank gear to minimize the amplitude of torsional vibration at the crank gear. Nallinger does not even appreciate the need to reduce the vibration transferred to the engine gear train. Therefore, Nallinger does not suggest that the inertia of the auxiliary flywheel should be selected to cause the nodal point to be located in the vicinity of the crank gear. In addition, the rear mounted flywheel is rigidly connected to the crankshaft and, therefore, this reference could not suggest selecting a resilient connection having a stiffness capable of optimally positioning the nodal point relative to the crank gear. U.S. Pat. No. 1,884,410 to Vincent discloses a crankshaft assembly including a flywheel rigidly mounted at the rear of the crankshaft. However, the Vincent assembly suffers the same drawbacks of discussed hereinabove with respect to Nallinger.
Another goal of engine designers is to effectively control gear rattle and noise. The gears are designed with looseness or backlash between the gear teeth to permit effective meshing, and prevent binding of the gears. This lash permits the gears to experience backlash and rattling in response to torsional vibrations and other effects, such as load changes, undesirably resulting in excessive gear noise and/or high loads on the gear teeth.
Consequently, there is a need for an engine drive train having a front mounted gear train which is capable of minimizing the torsional deflection experienced at the location of the connection of the gear train to the crankshaft.